1. Field of the Invention
The present invention relates to an exposure apparatus used in photolithography processing for producing a variety of micro devices, such as semiconductor devices, image pickup devices, liquid crystal display devices and thin film magnetic head, particularly relates to an exposure apparatus capable of correcting changes of automatic-focusing sensor system accurately at a high speed.
2. Description of the Related Art
In a photolithography processing of producing semiconductor devices, an exposure apparatus of a step-and-repeat type is widely used wherein a mask pattern is transferred on a wafer or glass plate (hereinafter, also referred to as a substrate) coated with photo resist. The step-and-repeat type exposure apparatus exposes a shot region by projecting at one time a reduced image of a mask pattern on the shot region on a wafer. When an exposure on one shot region is completed, the wafer is moved by a step and the next shot region is exposed. Since this process is repeated successively, it is called a step-and-repeat type.
While, in order to expand the exposure region of the mask pattern, a step-and-scan type exposure apparatus has been also developed wherein an exposure light from a illumination system is limited to be a slit shape (rectangular shape), and a mask and a wafer are scanned in synchronization with respect to a projection optical system in a state that a part of the mask pattern is reduced and projected on the wafer by using the slit light. The step-and-scan type exposure apparatus has both merits of a transfer mode of an aligner for transferring a full-scale pattern of overall surface of the mask on the wafer by one-time scan exposure and merits of the above transfer mode of the stepper.
In such kinds of exposure apparatuses, focusing is performed before exposure and during exposure. As a focusing method of a substrate coated with a photo-resist material, a method of using a so-called automatic focusing sensor is known.
In adjusting focus by the automatic focusing sensor, a height of a substrate surface (a position in the optical axis direction of a projection optical system) is detected by fixing a projector and a light receiver as an optical sensor on a frame on which a projection optical system is set, irradiating a light obliquely on the substrate surface from the projector, and receiving a reflected light from the substrate surface by the light receiver. Then, the position of the substrate stage in the optical axis direction is controlled by a signal therefrom to adjust the focus of the projection optical system on the substrate surface.
In the above focus adjusting method using the automatic focusing sensor, however, positions are not adjusted directly with respect to the substrate surface by detecting a focal position of an actual mask pattern image, but by measuring a gap between a control target position of the automatic focusing sensor and the substrate surface, and using and controlling it as a gap between the focal position of the projection optical system and the substrate surface. Thus, there is a disadvantage that an optical and mechanical drift of the automatic focusing sensor as it is becomes an error of adjusting focuses.
Namely, adjusting focus by the above automatic focusing sensor is performed not by directly measuring a gap between a focal position of the projection optical system and the substrate surface, but by measuring a gap between the control target position of the automatic focusing sensor and the substrate surface, and by making the control target position of the automatic focusing sensor and the substrate surface almost the same based on the measurement result so as to the substrate surface is adjusted to the focal position of the projection optical system.
Accordingly, if a relative position of the projection optical system and the automatic focusing sensor deviates for some reason, sometimes exposure is performed in a state the projection optical system does not focus on the substrate surface (an exposure surface of the substrate) even when a measured value of the gap detected by the automatic focusing sensor is within an appropriate range.
Therefore, a method of calibration of the automatic focusing sensor by detecting a focal position of the actual mask pattern image is sometimes applied, but there are many disadvantages in an actual use, such that detection of the focal position of the pattern image takes time, and high resolution cannot be expected in terms of its precision.
An object of the present invention is to provide an exposure apparatus capable of correcting changes of an automatic focus sensor system at high precision and a high speed.
Another object of the present invention is to provide an exposure apparatus capable of correctly setting a focal position (an image forming position of a pattern image) of a projection optical system and a substrate surface as a desired state when projecting a mask pattern image on to the substrate through the projection optical system.
According to the present invention, there is provided an exposure apparatus for transferring a pattern image formed on a mask on to a substrate through a projection optical system, comprising a substrate table which holds the substrate, a first sensor which measures a gap between the substrate surface held by a substrate table and a predetermined control target position, and a second sensor for measuring a distance between the projection optical system and the substrate table in an optical axis direction and correcting the control target position of the first sensor.
According to the present invention, even if the first sensor optically or mechanically moves with respect to the projection optical system, a distance between the projection optical system and the substrate table in the optical axis direction is measured by the second sensor, and the control target position of the first sensor is corrected based on the measurement result, thus, an exposure apparatus capable of correcting the moves of the first sensor at high precision and a high speed can be provided.